Читать книгу Some Salient Points in the Science of the Earth - Sir John William Dawson - Страница 13

Table of Contents

THE HISTORY OF THE NORTH ATLANTIC.

I

I had the pleasure of being present at the meeting of the British Association at Birmingham, in 1865: a meeting attended by an unusually large number of eminent geologists, under the presidency of my friend Phillips. I had the further pleasure of being his successor at the meeting in the same place, in 1886; and the subject of this chapter is that to which I directed the attention of the Association in my Presidential address. I fear it is a feeble and imperfect utterance compared with that which might have been given forth by any of the great men present in 1865, and who have since left us, could they have spoken with the added knowledge of the intervening twenty years.

The geological history of the Atlantic appeared to be a suitable subject for a trans-Atlantic president, and to a Society which had vindicated its claim to be British in the widest sense by holding a meeting in Canada, while it was also meditating a visit to Australia—a visit not yet accomplished, but in which it may now meet with a worthy daughter in the Australian Association formed since the meeting of 1886. The subject is also one carrying our thoughts very far back in geological time, and connecting itself with some of the latest and most important discussions and discoveries in the science of the earth, furnishing, indeed, too many salient points to be profitably occupied in a single chapter.

If we imagine an observer contemplating the earth from a convenient distance in space, and scrutinizing its features as it rolls before him, we may suppose him to be struck with the fact that eleven-sixteenths of its surface are covered with water, and that the land is so unequally distributed that from one point of view he would see a hemisphere almost exclusively oceanic, while nearly the whole of the dry land is gathered in the opposite hemisphere. He might observe that large portions of the great oceanic areas of the Pacific and Antarctic Oceans are dotted with islands—like a shallow pool with stones rising above its surface—as if the general depth were small in comparison with the area. Other portions of these oceans he might infer, from the colour of the water and the absence of islands, cover deep depressions in the earth's surface. He might also notice that a mass or belt of land surrounds each pole, and that the northern ring sends off to the southward three vast tongues of land and of mountain chains, terminating respectively in South America, South Africa, and Australia, towards which feebler and insular processes are given off by the antarctic continental mass. This, as some geographers have observed,[21] gives a rudely three-ribbed aspect to the earth, though two of the ribs are crowded together, and form the Eurasian mass or double continent, while the third is isolated in the single continent of America. He might also observe that the northern girdle is cut across, so that the Atlantic opens by a wide space into the Arctic Sea, while the Pacific is contracted toward the north, but confluent with the Antarctic Ocean. The Atlantic is also relatively deeper and less cumbered with islands than the Pacific, which has the highest ridges near its shores, constituting what some visitors to the Pacific coast of America have not inaptly called the "back of the world," while the wider slopes face the narrower ocean. The Pacific and Atlantic, though both depressions or flattenings of the earth, are, as we shall find, different in age, character, and conditions; and the Atlantic, though the smaller, is the older, and, from the geological point of view, in some respects, the more important of the two; while, by virtue of its lower borders and gentler slope, it is, though the smaller basin, the recipient of the greater rivers, and of a proportionately great amount of the drainage of the land.[22]

[21] Dana, "Manual of Geology," introductory part. Green, "Vestiges of a Molten Globe," has summed up these facts.

[22] Mr. Mellard Reade, in two Presidential addresses before the Geological Society of Liverpool, has illustrated this point and its geological consequences.

If our imaginary observer had the means of knowing anything of the rock formations of the continents, he would notice that those bounding the North Atlantic are, in general, of great age some belonging to the Laurentian system. On the other hand, he would see that many of the mountain ranges along the Pacific are comparatively new, and that modern igneous action occurs in connection with them. Thus he might see in the Atlantic, though comparatively narrow, a more ancient feature of the earth's surface; while the Pacific belongs to more modern times. But he would note, in connection with this, that the oldest rocks of the great continental masses are mostly toward their northern ends; and that the borders of the northern ring of land, and certain ridges extending southward from it, constitute the most ancient and permanent elevations of the earth's crust, though now greatly surpassed by mountains of more recent age nearer the equator, so that the continents of the northern hemisphere seem to have grown progressively from north to south.

If the attention of our observer were directed to more modern processes, he might notice that while the antarctic continent freely discharges its burden of ice to the ocean north of it, the arctic ice has fewer outlets, and that it mainly discharges itself through the North Atlantic, where also the great mass of Greenland stands as a huge condenser and cooler, unexampled elsewhere in the world, throwing every spring an immense quantity of ice into the North Atlantic, and more especially into its western part. On the other hand, he might learn from the driftage of weed and the colour of the water, that the present great continuous extension and form of the American continent tend to throw northward a powerful branch of the equatorial current, which, revolving around the North Atlantic, counteracts the great flow of ice which otherwise would condemn it to a perpetual winter.

Further, such an observer would not fail to notice that the ridges which lie along the edges of the oceans and the ebullitions of igneous matter which proceed, or have proceeded from them, are consequences of the settling downward of the great oceanic depressions, a settling ever intensified by their receiving more and more of deposit on their surfaces; and that this squeezing upward of the borders of these depressions into folds has been followed or alternated with elevations and depressions without any such folding, and proceeding from other causes. On the whole, it would be apparent that these actions are more vigorous now at the margins of the Pacific area, while the Atlantic is backed by very old foldings, or by plains and slopes from which it has, so to speak, dried away without any internal movement. Thus it would appear that the Pacific is the great centre of earth-movement, while the Atlantic trench is the more potent regulator of temperature, and the ocean most likely to be severely affected in this respect by small changes of its neighbouring land. Last of all, an observer, such as I have supposed, would see that the oceans are the producers of moisture and the conveyors of heat to the northern regions of the world, and that in this respect and in the immense condensation and delivery of ice at its north end, the Atlantic is by far the more active, though the smaller of the two.

So much could be learned by an extra-mundane observer; but unless he had also enjoyed opportunities of studying the rocks of the earth in detail and close at hand, or had been favoured by some mundane friend with a perusal of "Lyell's Elements," or "Dana's Manual," he would not be able to appreciate as we can the changes which the Atlantic has seen in geological time, and in which it has been a main factor. Nor could he learn from such superficial observation certain secrets of the deep sea, which have been unveiled by the sounding lead, the inequalities of the ocean basin, its few profound depths, like inverted mountains or table-lands, its vast nearly flat abyssmal floor, and the sudden rise of this to the hundred fathom line, forming a terrace or shelf around the sides of the continents. These features, roughly represented in the map prefixed, he would be unable to perceive.

Before leaving this broad survey, we may make one further remark. An observer, looking at the earth from without, would notice that the margins of the Atlantic and the main lines of direction of its mountain chains are north-east and south-west, and north-west and south-east, as if some early causes had determined the occurrence of elevations along great circles of the earth's surface tangent to the polar circles.

We are invited by the preceding general glance at the surface of the earth to ask certain questions respecting the Atlantic, (1) What has at first determined its position and form? (2) What changes has it experienced in the lapse of geological time? (3) What relations have these changes borne to the development of life on the land and in the water? (4) What is its probable future?

Before attempting to answer these questions, which I shall not take up formally in succession, but rather in connection with each other, it is necessary to state, as briefly as possible, certain general conclusions respecting the interior of the earth. It is popularly supposed that we know nothing of this beyond a superficial crust perhaps averaging 50,000 to 100,000 feet in thickness. It is true we have no means of exploration in the earth's interior, but the conjoined labours of physicists have now proceeded sufficiently far to throw much inferential light on the subject, and to enable us to make some general affirmations with certainty; and these it is the more necessary to state distinctly, since they are often treated as mere subjects of speculation and fruitless discussion.

(1) Since the dawn of geological science, it has been evident that the crust on which we live must be supported on a plastic or partially liquid mass of heated rock, approximately uniform in quality under the whole of its area. This is a legitimate conclusion from the wide distribution of volcanic phenomena, and from the fact that the ejections of volcanoes, while locally of various kinds, are similar in every part of the world. It led to the old idea of a fluid interior of the earth, but this seems now generally abandoned, and this interior heated and plastic layer is regarded as merely an under-crust, resting on a solid nucleus.[23]

[23] I do not propose to express any definite opinion as to this question, as either conclusion will satisfy the demands of geology. It would seem, however, that astronomers now admit a slight periodical deformation of the crust. See Lord Kelvin's Anniversary Address to Royal Society, 1892.

(2) We have reason to believe, as the result of astronomical investigations,[24] that, notwithstanding the plasticity or liquidity of the under-crust, the mass of the earth—its nucleus as we may call it—is practically solid and of great density and hardness. Thus we have the apparent paradox of a solid yet fluid earth; solid in its astronomical relations, liquid or plastic for the purposes of volcanic action and superficial movements.

[24] Hopkins, Mallet, Lord Kelvin, and Prof. G. H. Darwin maintain the solidity and rigidity of the earth on astronomical grounds; but different conclusions have been reached by Fisher, Hennesey, Delaunay, and Airy. In America, Hunt, Barnard and Crosby, Button, Le Conte and Wadsworth have discussed these questions. Bonney has suggested that a mass may be slowly mobile under long-continued pressure, while rigid with reference to more sudden movements.

(3) The plastic sub-crust is not in a state of dry igneous fusion, but in that condition of aqueo-igneous or hydrothermic fusion which arises from the action of heat on moist substances, and which may either be regarded as a fusion or as a species of solution at a very high temperature. This we learn from the phenomena of volcanic action, and from the composition of the volcanic and plutonic rocks, as well as from such chemical experiments as those of Daubrée, and of Tilden, and Shenstone.[25] It follows that water or steam, as well as rocky matter, may be ejected from the under-crust.

[25] Phil. Trans., 1884. Also Crosby in Proc. Boston Soc. Nat. Hist., 1883.

(4) The interior sub-crust is not perfectly homogeneous, but may be roughly divided into two layers or magmas, as they have been called; an upper, highly silicious or acidic, of low specific gravity and light-coloured, and corresponding to such kinds of plutonic and volcanic rocks as granite and trachyte; and a lower, less silicious or more basic, more dense, and more highly charged with iron, and corresponding to such igneous rocks as the dolerites, basalts, and kindred lavas. It is interesting here to note that this conclusion, elaborated by Durocher and Von Waltershausen, and usually connected with their names, appears to have been first announced by John Phillips, in his "Geological Manual," and as a mere common sense deduction from the observed phenomena of volcanic action and the probable results of the gradual cooling of the earth. It receives striking confirmation from the observed succession of acidic and basic volcanic rocks of all geological periods and in all localities. It would even seem, from recent spectroscopic investigations of Lockyer, that there is evidence of a similar succession of magmas in the heavenly bodies, and the discovery by Nordenskiöld of native iron in Greenland basalts, affords a probability that the inner magma is in part metallic, and possibly, that vast masses of unoxidised metals exist in the central portion of the earth.

(5) Where rents or fissures form in the upper crust, the material of the lower crust is forced upward by the pressure of the less supported portions of the former, giving rise to volcanic phenomena either of an explosive or quiet character, as may be determined by contact with water. The underlying material may also be carried to the surface by the agency of heated water, producing those quiet discharges which Hunt has named crenitic. It is to be observed here that explosive volcanic phenomena, and the formation of cones, are, as Prestwich has well remarked, characteristic of an old and thickened crust; quiet ejection from fissures and hydro-thermal action may have been more common in earlier periods and with a thinner over-crust This is an important consideration with reference to those earlier ages referred to in chapter second.

(6) The contraction of the earth's interior by cooling and by the emission of material from below the over-crust, has caused this crust to press downward, and therefore laterally, and so to effect great bends, folds, and plications; and these, modified subsequently by surface denudation, and the piling of sediments on portions of the crust, constitute mountain chains and continental plateaus. As Hall long ago pointed out,[26] such lines of folding have been produced more especially where thick sediments had been laid down on the sea-bottom, and where, in consequence, internal expansion of the crust had occurred from heating below. Thus we have here another apparent paradox, namely, that the elevations of the earth's crust occur in the places where the greatest burden of detritus has been laid down upon it, and where, consequently, the crust has been softened and depressed. We must beware, in this connection, of exaggerated notions of the extent of contraction and of crumpling required to form mountains. Bonney has well shown, in lectures delivered at the London Institution, that an amount of contraction, almost inappreciable in comparison with the diameter of the earth, would be sufficient; and that, as the greatest mountain chains are less than 1/600th of the earth's radius in height, they would, on an artificial globe a foot in diameter, be no more important than the slight inequalities that might result from the paper gores overlapping each other at the edges. This thinness of the crushed crust agrees with the deductions of physical science as to the shallowness of the superficial layer of compression in a cooling globe. It is perhaps not more than five miles in thickness. A singular proof of this is seen by the extension of straight cracks filled with volcanic rock in the Laurentian districts of Canada.[27] The beds of gneiss and associated rocks are folded and crumpled in a most complex manner, yet they are crossed by these faults, as a crack in a board may tear a sheet of paper or a thin veneer glued on it. We thus see that the crumpled Laurentian crust was very thin, while the uncrushed sub-crust determined the line of fracture.